Generally, semiconductor devices are used in a variety of electronic applications, such as computers, cellular phones, personal computing devices, and many other applications. Home, industrial, and automotive devices that in the past comprised only mechanical components now have electronic parts that require semiconductor devices, for example.
Semiconductor devices are manufactured by depositing many different types of material layers over a semiconductor workpiece or wafer, and patterning the various material layers using lithography. The material layers typically comprise thin films of conductive, semiconductive, and insulating materials that are patterned and etched to form integrated circuits (IC's). There may be a plurality of transistors, memory devices, switches, conductive lines, diodes, capacitors, logic circuits, and other electronic components formed on a single die or chip, for example.
Optical photolithography involves projecting or transmitting light through a pattern made of optically opaque areas and optically clear areas on a mask or reticle. For many years in the semiconductor industry, optical lithography techniques such as contact printing, proximity printing, and projection printing have been used to pattern material layers of integrated circuits. Projection printing is commonly used in the semiconductor industry using wavelengths of 248 nm or 193 nm, as examples. At such wavelengths, lens projection systems and transmission lithography masks are used for patterning, wherein light is passed through the lithography mask to impinge upon a semiconductor wafer or workpiece.
There is a trend in the semiconductor industry towards scaling down the size of integrated circuits, to meet demands of increased performance and smaller device size. As the minimum feature sizes of IC's are decreased, the semiconductor industry is exploring the use of alternatives to traditional optical lithography techniques, in order to meet the demand for decreased feature sizes in the industry. For example, short wavelength lithography techniques, Scattering with Angular Limitation in Projection Electron-beam Lithography (SCALPEL), other non-optical lithographic techniques, and immersion lithography are under development as replacements for traditional optical lithography techniques.
In immersion lithography, a gap between the last lens element in the optics system and a semiconductor wafer is filled with a liquid, such as water, to enhance system performance. The presence of the liquid enables the index of refraction in the imaging plane, and therefore the numerical aperture of the projection system, to be greater than unity. Thus, immersion lithography has the potential to extend exposure tool minimum feature sizes down to about 45 nm or less, for example.
As features of semiconductor devices become smaller, it becomes more difficult to pattern material layers because of diffraction and other effects that occur during the lithography process. In particular, lithography techniques used to pattern the various material layers become challenging as device features shrink.
Thus, what are needed in the art are improved lithography techniques and lithography masks that are capable of patterning smaller features of semiconductor devices.